A significant step has been made in extending the Northern Underground line to Battersea with the start of tunnelling work.
Two giant tunnel boring machines (TBMs), named Helen and Amy, were lowered 20 metres below the ground in February in preparation for the £1.2 billion project, the first major Tube line extension since the Jubilee line in the late 1990s.
Helen has now begun her 3.2km tunnelling journey to connect Battersea to Kennington, via Nine Elms, to create the first of the underground tunnels that will extend the Charing Cross branch of the Northern line. Amy is expected to follow in one month.
Each 650-tonne machine will run for 24 hours a day and will bore up to 30 metres of new tunnel a day.
The TBMs were built by NFM Technologies in Le Creusot in central France and are the length of the pitch at Wembley Stadium. They were shipped to London earlier this year and reassembled in Battersea.
More than 300,000 tonnes of earth will be excavated as the tunnelling machines advance to make the 3.2km tunnels.
The spoil will be transferred from the tunnels up to barges on the River Thames where it will be transported by barge to Goshems Farm in East Tilbury, Essex, to create arable farmland.
Almost 20,000 precast concrete segments will be put in place to form rings to line the tunnels.
According to tunnelling tradition, tunnel boring machines cannot start work until they are given a name. Following a vote by local school children, the machines were named in honour of the first British astronaut, Helen Sharman, and British aviation pioneer, Amy Johnson, who was the first female pilot to fly solo from Britain to Australia.
The Northern line extension is set to support around 25,000 new jobs and the construction of more than 20,000 new homes.
Tunnelling is expected to take around six months to complete. Transport for London is aiming to have the extension complete by 2020.